Horse-Breeding Soundness, Part 2

A regular evaluation on your stallion is a wise investment.

From The American Quarter Horse Journal

It helps you know how to manage your stallion to get the output you need. Journal photo.

If you missed our last article, no worries, we will fill you in: Read “Horse-Breeding Soundness, Part 1.” The article refers to a formal breeding soundness evaluation on a stallion. The BSE begins with a physical exam: a check for lameness, pulse, respiration and body condition. Toward the end of physical examination, after collection, testicular palpations allow examiners to check for cryptorchidism. Also considered is scrotal volume, sperm production and a bacterial culture. Behavior is the next key thing to take into account. You do not know the true behavior until you see the stallion in action.

Dr. Jason Bruemmer, professor in the department of animal sciences at Colorado State University. Stallion behavior, physiology and management are his primary research fields, and he directs the stallion services offered at CSU Equine Reproduction Laboratory says, “For example, you might use a gray tease mare and not know or realize that he has been kicked by a gray mare and that is what is taking so long.” The main takeaway from that portion is to start young and make sure the stallion has a consistent handling program to make it easy on him and yourself.

Semen Evaluation

There are two typical protocols for collecting semen for the BSE.

“The first is to collect a stallion every day for a week to 10 days,” Dr. Bruemmer says. “That will answer most of the questions we need to know about him.”

Because of the cost involved, most owners don’t do that.

“The other is to make sure the stallion has been sexually rested for a week and then collect him twice in a given day,” Dr. Bruemmer says. “We then compare the quality of the second ejaculate to that of the first.

“In a normal stallion, we expect to find between 20 to 70 percent of the total numbers in the second ejaculate as we did in the first; that indicates he’s depleting his extra gonadal reserves normally.”

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The evaluation of the semen itself involves a number of parameters: 1. measuring total liquid volume of the ejaculate with a graduated cylinder; 2. using a densimeter to measure the concentration of sperm cells per milliliter; 3. estimating the sperm total and progressive motility under a microscope, either by eye or using a motility analyzer; and 4. looking at the cells’ morphology via microscope slide stain.

“The sperm cells should have one head, a long midpiece and a straight tail,” Dr. Bruemmer says. “One of the most common abnormalities is a bent or curled tail, which affects the sperm’s progressive motility.

“On the morphological stain, a normal cell won’t stain; if the cell takes up the stain, you know its membrane is damaged.”

The goal is to get an idea of the sperm numbers the horse produces from the volume and concentration; and to get an idea of the sperm quality by looking at the cells’ progressive motility (the percentage of cells moving in straight lines) and physical structure in the morphology.

Depending on the owner’s plans, the BSE might also involve a test cool and/or a test freeze on the stallion’s semen.

“A common misconception is that if a horse has good-looking semen that it cools or freezes well,” Dr. Bruemmer says. “Until we do a test freeze or test cool, we don’t know.”

For the test, the lab performs a standard BSE, then gives the horse a day’s rest, and on the third day collects the stallion once and splits the ejaculate for the test cool and freeze.

“Just because a stallion has semen that cools well doesn’t mean it will freeze well, and vice versa,” Dr. Bruemmer adds.


“The tricky part is what makes a stallion pass or fail a BSE,” Dr. Bruemmer says. “When I put one of these together, it’s not like any pre-purchase exam you’d get on a horse. There’s no such thing as a perfect horse: You simply state, ‘This is what we found on this day.’ ”

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He adds that the BSE measures potential indicators of fertility and should not be taken as a guarantee either way.

“We’ve got examples of stallions with the best-looking semen in the world but with poor fertility; and we’ve got stallions here that I have no idea how they’re getting mares pregnant on a regular basis.

“There is no better test than to breed mares,” he says. “If he gets mares in foal, that’s your actual pass/fail answer.”

Passing or failing can also be dependent on who owns the stallion and how the owners want to use him. For example, a horse could pass in a live-cover program, but fail in a frozen semen program.

Predicting Production

Combining the stallion’s total scrotal volume with the numbers measured in volume, concentration and progressive motility gives owners a valuable tool in managing a stallion’s book.

Dr. Bruemmer explains that there are established, expected sperm numbers of what a normal stallion produces throughout the year. The highest sperm production hits in June.

“The idea is that you can do a BSE on a stallion any time of the year and still estimate what his sperm output will be in a peak month, assuming he has been in ambient light,” Dr. Bruemmer says. “Then with that information, we can predict the daily sperm output.

“Some of the farms we’ve worked with don’t actually book mares until they’ve had evaluations of their stallions, which is brilliant,” Dr. Bruemmer says. “The BSE allows us to estimate a stallion’s book so you can better manage your business.

“It helps you know how to manage your stallion to get the output you need.”